The present invention relates to methods and apparatus for removing heavy metals, other cations and anions from aqueous and non-aqueous media.
Existing systems for removing heavy metals include direct current electrolytic processes. One of the major deficiencies of direct current electrolytic processes is their inability to efficiently remove low level concentrations of toxic anions and cations from aqueous streams. There is a major growing environmental problem of release of contaminants into ponds, lakes, surface water, subsurface water, rivers and oceans. Toxic heavy metals are not biodegradable and usually accumulate in the environment. One major type of contaminant release is that of heavy metals from the mining and smelting industries. Runoff water from operating mines and abandoned mine sites contain both low levels and high levels of toxic heavy metals. Those sites often discharge the accumulated metal wastes into clean streams, thereby contaminating the streams. Severe environmental impacts result. A stop gap technology most widely used to combat that particular problem involves the use of lime for raising the pH of aqueous solutions and for precipitating the heavy metals. A large volume of sludge is produced in which the metals are not concentrated sufficiently to be of interest to a metals reclaimer. That sludge is toxic and accumulates wherever the limiting process is used.
Existing cation or anion ion exchange units collect heavy metals and other cations on the resin surface until breakthrough of one those cations or anions is observed. Conventional regeneration of the resin bed usually occurs by passing 1-2 bed volumes of either strong acid, strong base or salt solution through the resin bed. That is followed by 4-5 bed volumes of flushing and rinse fluids. Those resin beds are typically downflow in service and upflow in the regeneration and rinse cycles. Conventional regeneration procedures produce substantial quantities of hazardous fluids. Metal reclaimers are not interested in reclaiming the metals from the regeneration fluids as they are typically very toxic and the concentration of metals is too low for the reclaimer to make a profit.
Needs exist for methods and apparatus for removing metals, cations and anions from water that are environmentally friendly and economically feasible.
Concerns regarding the presence of heavy metals in water supplies are rapidly increasing. Elements such as cadmium, mercury, silver, chromium, lead, copper and zinc exhibit toxicity in humans. The promiscuous release of heavy metal and toxic anions into the environment pose great dangers because of their toxicity and relative accessibility.
Major sources by which heavy metals enter aquatic environments include the metal processing, metal finishing and plating industries and leachate runoffs from toxic metal dumps. The major toxic heavy metals generated by industrial and mining industries and found in waste water include copper, cadmium, nickel, lead, zinc, chromium, mercury and the radioactive elements radium, thorium and uranium. Related chelating agents are also found in the waste water, including ethylenediaminetetraacetic acid, nitrilotriacetic acid, citrate, tartrate, gluconic acid and the like. More than 13,000 corporations are involved with aspects of metal finishing and electroplating.
A number of specialized processes have been developed to remove heavy metals from industrial waste waters. Processes that have been investigated include: chemical precipitation, ion exchange, solvent extraction, cementation, coagulation/flocculation, complexation, adsorption, electrochemical operations, biological operations, filtration, evaporation and reverse osmosis/ultra filtration.
Current state-of-the-art methods for treating plating wastes from facilities employ precipitation treatment with conventional hydroxide precipitation of a mixed waste water in a single reactor. Nearly 75 percent of existing plating facilities employ precipitation treatment (primarily hydroxide treatment) as the treatment scheme for removal of heavy metals from solutions. It is the most widely used process industrially.
In the hydroxide precipitation process, heavy metals are precipitated by adding an alkali, such as caustic soda or lime to adjust the waste water pH to the point where the metal exhibits its minimum solubility. The metals precipitate as metal hydroxides and can be removed by flocculation and sedimentation. The extent of the precipitation depends on the solubility product (K.sub.sp) of the metal hydroxide and the equilibrium constants, K.sub.I 's, of the metal-hydroxyl complexes. The metal-hydroxide precipitates can be removed by adequate solid-liquid separation processes such as sedimentation and filtration. The effectiveness of separation is highly dependent on the physical properties (size, density, etc.) of those metal hydroxide precipitates. Wide-spread acceptance of the hydroxide treatment is due to its relative simplicity, low cost of precipitant (lime) and ease of automatic pH control. Sulfide precipitation is an alternative process for removal of heavy metals due to the low solubilities of the sulfides. Both processes produce toxic sludges which must be reclaimed or require disposal. The sulfide process has the potential to generate toxic hydrogen sulfide gas and there are environmental concerns associated with the toxicity of sulfides.
Limitations of the hydroxide process include the following: precipitates resolubilize if pH changes; mixed metal wastes require different pH conditions for metals having different precipitation solubilities; the presence of complexing agents has an adverse effects on metals removal; chromium (VI) is not removed by the hydroxide technique; hydroxide sludge quantities are substantial; hydroxide sludges are difficult to dewater due to the amorphous structure; and little metal hydroxide precipitation occurs below pH of 6.